1. Field of the Invention
The present invention generally relates to a technology for Mobile IP (Internet Protocol)-compliant mobile communications, and particularly relates to a wireless communications system, and a relaying apparatus and a mobile terminal that are for use therein.
2. Description of the Related Art
With the proliferation of networking technologies, research and development activities regarding Internet protocols and architectures are in progress. More specifically, for an Internet protocol (the so-called Mobile IP) for assisting a communications terminal in moving, studies by such organizations as the IETF (Internet Engineering Task Force) are being carried out toward standardization.
FIG. 1 is one example of a wireless-communications system which is compliant with such Mobile IP as described above. A wireless communications system 100 has a first router 102, a second router 104, a third router 106 and a home agent 108 that route a communications signal to one another via a network 101 such as the Internet. Moreover, the wireless-communications system 100 has multiple wireless base-stations, the wireless base-stations respectively forming multiple wireless zones which collectively make up a wide wireless service area. In a wireless zone 112 of a wireless base-station 110 connected to the home agent 108, a home link for a mobile terminal 114 is formed. This mobile terminal 114 has a specific IP address (a home address) which does not change when moving between the wireless zones. The corresponding relationship between the home address and a care-of address to be described below is managed at the home agent 108.
A first wireless base-station 116 connected to the first router 102 wirelessly communicates with a mobile terminal 114 within a wireless zone 118. A second wireless base-station 120 connected to the second router 104 wirelessly communicates with the mobile terminal 114 within a wireless zone 122. It is assumed that the third router 106 is connected to a correspondent node 124 in communications with the mobile terminal 114.
Next, an overview of transmitting and receiving a communications signal in this wireless-communications system is provided. It is assumed that the mobile terminal 114 having a home address HA that is located within the wireless zone 118 of a first wireless base-station 116 communicates with the correspondent node 124. The mobile terminal 114 obtains a care-of address CoA1 within the wireless zone 118, while the corresponding relationship between the care-of address CoA1, which may change when the mobile terminal moves, and the fixed home address HA is kept track of by the home agent 108. The correspondent node 124, in a case of knowing the care-of address CoA1 when transmitting a communications signal to the mobile terminal 114 within the wireless zone 118, designates CoA1 as header information indicating where the communications signal is destined for so as to route the communications signal by means of the third router 106 to the first router 102, causing the communications signal to arrive at the mobile terminal 114 under the control of the first router 102. On the other hand, the correspondent node 124, in a case of not knowing the care-of address CoA1 and knowing only the home address HA, designates HA as the information indicating where the communications signal is destined for so as to route the communications signal from the third router 106 by means of the home agent 108 to the first router 102, causing the communications signal to arrive at the mobile terminal 114 under the control of the first router 102.
Once the mobile terminal 114 moves to enter a neighboring wireless zone 122, the mobile terminal 114 obtains another care-of address CoA2 to be used within the wireless zone 122. It is desirable that this care-of address CoA2 be used in case the mobile terminal 114 shifts to the wireless zone 122 and communicates therein. Thus, after shifting from the one wireless zone to another, the mobile terminal 114 transmits to the home agent 108 an instruction signal indicating that a care-of address for associating with the home address HA should be changed from CoA1 to CoA2. Moreover, the mobile terminal 114 transmits to the first router 102, by which the mobile terminal 114 was being controlled originally, an instruction signal indicating that a communications signal destined for CoA1 should be encapsulated as destined for CoA2 so as to transfer to CoA2. Furthermore, the mobile terminal 114 as needed transmits also to the correspondent node 124 an instruction signal indicating a change in the care-of address. These instruction signals are called Binding-Update signals.
The correspondent node knowing only the home address HA setting where the communications signal is destined for as the home address HA makes it possible to transmit the communications signal to the mobile communications terminal 114 through the home agent 108 and the second router 104. The communications signal routed for the previous care-of address CoA1, while being routed to the first router 102, is also encapsulated as destined for CoA2 so as to be transmitted to CoA2 so that the signal arrives at the second router 104 in this case also. Thus, regardless of where in the network the mobile terminal moving between the wireless zones is connected to, it becomes possible for the correspondent node to communicate using the same IP address (more specifically, the home address). It is noted that, for the wireless-communications system of the type as described above, there is, for example, a disclosure in the Japanese Publication of Patent Applications 2000-332825A.
On the other hand, a communications signal in a mobile communications environment is influenced by fading which causes a random change in the envelope level of the radio wave. A technology called diversity is often used in order to reduce the influence of fading. Generally, diversity provides at the transmitting side multiple fading waves which change in mutually independent manners so as to be combined at the receiver side. Of the different types of diversity, there is one type such that transmissions are made from each of two or more wireless base-stations as diversity branches (representing nodes to be the transmitters when implementing site diversity) so as to try to improve the transmission quality of a communications signal in the vicinity of a border of wireless zones. It is noted that, for the site diversity using such multiple wireless base-stations as described above, there is, for example, a disclosure in an international publication pamphlet number WO 95/32594.
Up to now, site diversity has not been implemented in a Mobile-IP communications environment such as described above. As a communications signal is routed to one destination (an IP address) indicated in the header of the communications signal in Mobile-IP routing, even if one were to implement diversity, it would be difficult to implement the diversity branches themselves, for instance.